1. Field of the Invention
The present invention relates generally to an ultrasonic system for detecting the presence of air in a fluid line, and more particularly to a self-test procedure for ensuring that any faults in the ultrasonic air-in-line detector which do not fail safe are automatically detected by periodically performing a self-test procedure.
In the past there have been two primary techniques which have been used to deliver drugs which may not be orally ingested to a patient. The first such technique is through an injection, or shot, using a syringe and needle which delivers a large dosage at relatively infrequent intervals to the patient. This technique is not always satisfactory, particularly when the drug being administered is potentially lethal, has negative side effects when delivered in a large dosage, or must be delivered more or less continuously to achieve the desired therapeutic effect. This problem results in smaller injections being given at more frequent intervals, a compromise approach not yielding satisfactory results.
Alternatively, the second technique involves administering a continuous flow of medication to the patient, typically through an IV bottle. Medication may also be delivered through an IV system with an injection being made into a complex maze of IV tubes, hoses, and other paraphernalia. With drop counters being used to meter the amount of bulk fluid delivered, many medications still end up being administered in a large dosage through an injection into the IV lines, although the medications may be diluted somewhat by the bulk fluid.
As an alternative to these two techniques of administering medication to a patient, the relatively recent addition of medication infusion pumps has come as a welcome improvement. Medication infusion pumps are utilized to administer drugs to a patient in small, metered doses at frequent intervals or, alternatively, in the case of some devices, at a low but essentially continuous rate. Infusion pump therapy may be electronically controlled to deliver precise, metered doses at exactly determined intervals, thereby providing a beneficial gradual infusion of medication to the patient. In this manner, the infusion pump is able to mimic the natural process whereby chemical balances are maintained more precisely by operating on a continuous time basis.
One of the requirements of a medication infusion system is dictated by the important design consideration of disposability. Since the portion of the device through which medication is pumped must be sterile, in most applications of modern medication infusion equipment some portions of the equipment are used only once and then disposed of, typically at regular intervals such as once daily. It is therefore desirable that the fluid pump portion of the infusion pump device be disposable, with the fluid pump being designed as an attachable cassette which is of inexpensive design, and which is easily installable onto the main pump unit.
It will be perceived that it is desirable to have a simple disposable cassette design to minimize the cost of construction of the cassette, using the minimum number of parts necessary in the design of the cassette. The design of the cassette must be mass producible, and yet result in a uniform cassette which is capable of delivering liquid medication or other therapeutic fluids with a high degree of accuracy. The cassette should include therein more than just a fluid pump; other features which have formerly been included in peripheral devices may be included in the cassette.
Such a system has been disclosed in all of the above-identified previously filed related applications. Of these applications, U.S. Ser. No. 128,121, entitled "Air-In-Line Detector for a Medication Infusion System," is hereby incorporated herein by reference.
An essential function of a medication infusion system is to avoid the infusion of fluid containing more than a minimal amount of air bubbles therein. Although steps may be taken to minimize the possibility of air bubbles being contained in a fluid which is to be infused to a patient, it is essential to monitor the fluid line before it reaches the patient to ensure that air bubbles remain in the fluid which is to be infused are detected. The detection of air bubbles in all fluids which are to be infused is therefore a critical design requirement.
One type of air-in-line detector which has been used in the past is an ultrasonic detector, which places an ultrasonic transmitter on one side of a fluid line and an ultrasonic receiver on the other side of the fluid line. Fluid is a good conductor of ultrasonic energy while air or foam is not. Accordingly, if there is an air bubble in the fluid line between the transmitter and the receiver, the signal strength will be greatly attenuated, and the presence of the bubble will be indicated. Examples of ultrasonic air-in-line detectors include U.S. Pat. No. 4,764,166, to Spani, and U.S. Pat. No. 4,821,558, to Pastrone et al.
It will at once be realized by those skilled in the art that the ultrasonic air-in-line detector is a critical component of the medication infusion system. As such, all possible failures of the ultrasonic air-in-line detector must either fail-safe or be promptly detected. An example of a fail-safe condition is the failure of one of the transducers, in which case the system will indicate that air is present in the fluid line even when fluid is present. Other than fail-safe failures are those which would indicate that fluid is present in the fluid line when in fact air is present. Such failures should be promptly detected by the system, although the references cited above are silent as to any apparatus or procedure for detecting non fail-safe failures.
There are two known non-fail-safe conditions known to occur in an ultrasonic air-in-line detector. The first of these non-fail-safe failures is when the output of the receiver is stuck high. This occurs typically because there is a short in the receiver to V.sub.CC. If this situation occurs, the output of the ultrasonic air-in-line detector will remain high indicating the presence of fluid in the fluid line even when air is in the fluid line.
The second known fail-safe failure is when there is electrical coupling between the transmitter and either of the ultrasonic receiver transducer, the receiver circuitry, or the digital output circuitry following the receiver circuitry. This may occur due to situations such as shorts, stray capacitance, or stray inductance. Such electrical coupling may have a bandwidth anywhere from DC to MHz. Either such electrical coupling or a receiver which is stuck high will thus cause the ultrasonic air-in-line detector system to indicate that there is fluid in the line when in fact there is air in the line.
It is therefore the primary objective of the present invention to provide a self-test system which will detect all such non-fail-safe occurrences. Thus, the self-test system must detect the occurrence of a receiver output stuck high and provide an alarm and shut down the pumping system. The self-test system must also detect the occurrence of electrical coupling which causes a false indication of the presence of fluid in the fluid line, and provide an alarm and shut down the pumping system.
Such a self-test must be performed periodically, and sufficiently often to ensure that such a failure will be detected promptly before air can be pumped into the patient. The self-test system must use as few additional components as possible, and require no modification to the cassette, yet which afford the highest degree of accuracy in detecting a system fault. The system of the present invention must provide all of these advantages and overcome the limitations of the background art without incurring any relative disadvantage.